Nucleic acids such as siRNA (small interfering RNA), miRNA (micro RNA), shRNA (short hairpin RNA or, small hairpin RNA) expression vector, or, antisense oligonucleotides are nucleic acids which induce inhibition of sequence-specific gene expression in vivo, and are known as nucleic acid medicines.
Among these nucleic acid medicines, in particular, siRNA has attracted attention. siRNA is a double stranded RNA having 19-23 base pairs, and induces inhibition of sequence-specific gene expression called RNA interference (RNAi).
However, although siRNA is chemically-stable, there are problems in therapeutic applications. For example, it is degraded by RNase (ribonuclease) in plasma, and it hardly penetrates the cell membrane alone (e.g., Patent Document 1).
To address the problem, it is known that an encapsulated siRNA is protected from degradation in plasma by encapsulating siRNA into fine particles containing cationic lipids, and it is possible to penetrate lipophilic cell membranes. As lipid particles containing cationic lipids, a lipid formulation containing certain cationic lipids, which are prepared by an extrusion method or an in-line mixing process, has been proposed (e.g., see Patent Document 1.).
The lipid formulation described in Patent Document 1 is a nucleic acid-lipid particle composition which is obtained by encapsulating a polymer such as nucleic acids in fine particles, and Patent Document 1 discloses that it is possible to introduce nucleic acids into cells.
Further, various compounds as cationic lipids which are expected to be used in application of nucleic acid medicines have been developed (e.g., Patent Documents 2-6, and Non-Patent Document 1). However, it is known that there are problems in physical stability in that the particle size of lipid nanoparticles (LNPs) containing cationic lipids with an asymmetrical structure increases during storage (non-Patent Document 2).
For solving the problem, Non-Patent Document 2 discloses that it is possible to suppress an increase of particle size by selectively-using certain phospholipids in LNPs containing cationic lipids having an asymmetric structure.    [Patent Document 1] Japanese Unexamined Patent Publication No. 2012-530,059    [Patent Document 2] International Publication No. 2012/040,184    [Patent Document 3] International Publication No. 2013/059,496    [Patent Document 4] International Publication No. 2010/144,740    [Patent Document 5] US Publication No. 2013/0178541    [Patent Document 6] U.S. Pat. No. 8,158,601    [Non-Patent Document 1] Angew. Chem. Int. Ed. 2012, 51, 8529-8533    [Non-Patent Document 2] Mol. Pharmaceutics 2014, 3, 11 (11), 4143-53